On the Shear Degradation of Lost-Circulation Materials

Abstract

Summary Lost-circulation materials (LCMs) are widely used to mitigate fluid loss when drilling permeable zones. Their effectiveness, however, generally declines with circulation time, and this decline is linked to the reduction in the average size of the solids components, or “shear degradation.” In this paper, dimensional analysis and first-principle physics are used to frame those mechanisms into a scientific definition that directly connects the progressive LCM size reduction to operational parameters such as the densities of particles and suspending fluid, the size of particles, and the fluid viscosity. The introduction of large-sized materials in the drilling-mud circulation system has become a common practice during the past few decades for the mitigation of lost circulation while drilling in permeable intervals. Solids with an average diameter larger than 100 μm are carried with the drilling mud and, when fractures occur, they deposit in the fractures or at the opening of the fractures, successfully blocking the discharge of fluid out of the wellbore. The effectiveness of such material was observed, however, to decline over time as the drilling mud circulates through the mud pumps, the drillstring, the bit nozzles, the wellbore annulus, and the mud-recycling system. The decline over time in the effectiveness of the so-called LCMs and loss-prevention materials (LPMs) during drilling is widely linked to the degradation of the LCM/LPM components that have an average size that drops with circulation time. A large amount of literature in the petroleum industry is dedicated to the investigation of the issue, and numerous experimental studies have attempted to quantify the LCM degradation by reproducing the root phenomena in the laboratory. The concept of “shear degradation” has thus become a synonym for such studies, and it is widely used for the selection of materials to be used in drilling operations. In the present paper, a harder look is taken on the very concept of “shear degradation” and on its effectiveness in capturing the progressive size reduction of LCM during circulation. By use of dimensional analysis, the tendency of a material to degrade can be determined in advance, whereby the density of the particles and suspending fluid, the size of particles, and the fluid viscosity are examined as the governing parameters. Understanding the underlying physics enables the selection of a more-shear-resistant engineered-particle drilling fluid, regardless of its application.